Fix for build auto-cancel: limit to only PRs

Multiple `main` builds were also cancelled.

See rubocop/rubocop#11414 for more context.

Follow-up to #2652

Author: pirj

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ on:
 permissions:
   contents: read
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.github.head_ref || github.run_id }}
   cancel-in-progress: true
 
 jobs: